This application pertains generally to the field of nonreciprocal optical devices such as optical isolators and optical circulators, which are constructed from materials that exhibit the optical Faraday effect. Optical isolators are commonly used to overcome the instability in semiconductor light sources caused by reflected light. Optical circulators may be used in two-way fiber optic communication systems and in other applications. In particular, this application pertains to quasi-achromatic isolators and circulators using total internal Fresnel reflection to simplify their construction.
Nonreciprocal optical devices such as isolators and circulators may be constructed from materials that exhibit the optical Faraday effect. This effect is a circular birefringence that arises from the presence within the material of a magnetization resulting from an externally applied magnetic field or from an internal spontaneous magnetization due to ferromagnetic or ferrimagnetic ordering that may be held in a saturated state by an externally applied magnetic field. In either case, it manifests itself as an optical rotatory effect upon light propagating through the material along the direction of magnetization. It is nonreciprocal in that the sense of rotation of the axes of polarization depends on the polarity of the magnetization relative to the direction of propagation.
Optical signals transmitted through fiber optic waveguides are being used for telecommunications to an ever increasing extent. They are generated by laser diodes of various types that often operate at wavelengths in the 1.28 to 1.60 .mu. range. Some of these lasers, especially those of the so-called distributed feedback construction, are somewhat sensitive to light returning on their output fiber whether it be from reflections of their own emissions or from another source. An optical isolator, which is a nonreciprocal two port device that passes light in one direction and absorbs light in the opposite direction, is often necessary to obtain optimum operation from these laser diode sources.
The optical circulator is a more generally applicable nonreciprocal four port device. As with the isolator, light entering the first port passes out the second port, but light entering the second port is not absorbed, and instead passes out the third port. Similarly, light entering the third port passes out the fourth port, and light entering the fourth port passes out the first port. Thus, by using any two adjacent ports a circulator can function as an isolator, but it also has the potential of permitting optical fiber transmission lines to be operated in a bidirectional mode with signals at the same or different wavelengths traveling in opposite directions simultaneously.
Basic to the operation of both optical isolators and circulators is the 45.degree. Faraday rotation element which is usually composed of glass or a single crystal transparent over the desired wavelength range. Opposing parallel optical facets surround the active region which is within an externally applied axial magnetic field provided by adjacent permanent magnets or by a current carrying solenoid. The field strength required to obtain 45.degree. of rotation depends on the Verdet constant of the element material. Suitable materials include diamagnetic glasses especially those with a high lead oxide content, paramagnetic glasses or cubic crystals containing ions such as trivalent cerium or terbium, and ferrimagnetic oxide crystals such as yttrium iron garnet. The latter, commonly known as YIG, is especially useful in the 1.28 .mu. to 1.60 .mu. wavelength range where many optical fiber systems operate.
In its simplest form an optical isolator consists of an input plane polarizer, a 45.degree. Faraday element with its associated axial field magnet, and an output plane polarizer with its polarization axis rotationally orientated at 45.degree. relative to that of the input polarizer. A compact isolator of this type using a YIG crystal has been described in the literature. Input light must be plane polarized to pass through the input polarizer after which its plane of polarization is rotated 45.degree. by the Faraday element so that it can pass through the output polarizer. If the propagation direction is reversed, the Faraday element will rotate -45.degree. and the light passed through it will be absorbed in the output polarizer. A similar optical circulator, also using a YIG crystal, but with input and output polarization beam splitters instead of plane polarizers has also been described in the literature. Both devices require specific states of plane polarization at their ports to function optimally.
The degree of isolation obtainable with either of these nonreciprocal devices is limited by deviations of the Faraday element rotation from its nominal 45.degree.. The element is designed for some nominal wavelength, and in general it will have a greater rotation at shorter and a lesser rotation at longer wavelengths. Also, some Faraday elements such as YIG are temperature sensitive so the rotation will change due to temperature variations. Various techniques have been used to improve the degree of isolation by minimizing these deviations from 45.degree. rotation. In the case of YIG, gadolinium substitution for part of the yttrium lowers the temperature coefficient of the rotation, but at the expense of its magnitude. The wavelength dependence can be partially compensated by a second element having -45.degree. of reciprocal type rotation Such an element can be made from an optically active crystal. The two-element combination between crossed polarizers would be used as an isolator. For one direction of propagation the opposite rotations would always sum to zero if they had identical wavelength dependences. But for the opposite direction of propagation both elements would have -45.degree. of rotation which would sum to -90.degree. with a doubled wavelength variation. The isolator would therefore have a wavelength dependent insertion loss.
The cross-referenced application, discloses a quasi-achromatic configuration of two Faraday elements and five birefringent plates which when suitably oriented between two linearly polarizing elements would constitute an optical isolator or circulator. The nominal Faraday rotations of the two elements are 45.degree. and 90.degree. at a center design wavelength about which the devices are to operate. Changes in these rotations due to either wavelength or temperature variations compensate one another because of their coupling by the birefringent plates. In this way a higher degree of isolation is obtained over a wider optical bandwidth than would be possible in a device using a single 45.degree. Faraday element. A group of three plates is used between the two Faraday elements and two more follow them to give the required polarization transformations which must themselves be quasi-achromatic over the desired wavelength range.